This invention relates to an economical ultraviolet-radiation stabilizer and flame retardant system for use in acrylonitrile-butadiene-styrene based thermoplastic formulations.
It is well recognized in the thermoplastic art that acrylonitrile-butadiene-styrene copolymers (ABS) are sensitive to UV radiation, which radiation is a component of sunlight and some artificial lighting. When ABS is combined with brominated aryl or cycloalkyl flame retardants in thermoplastic formulations, articles made from such formulations are especially sensitive to UV radiation. Sensitivity to UV radiation can be evidenced by discoloration and/or embrittlement of the articles.
The art has approached this problem from two directions. The most basic approach is to mask the discoloration of the article by loading the thermoplastic formulation with white pigment, e.g. ZnO, ZnS, TiO.sub.2, or CaCO.sub.3. To be effective, these loadings are substantial, say about 3 to 10 weight percent based upon the total weight of the thermoplastic formulation. While apparent color degradation is diminished, the use of the pigment at the needed loadings can adversely effect the article's properties.
The second approach does not rely on masking color degradation, but rather attempts to render the UV radiation impotent in its color degrading and/or embrittlement effect. This is accomplished by incorporating UV absorbers, excited state quenchers, or a mixture thereof in the thermoplastic formulation. Exemplary of compounds which have good UV absorption qualities are the 2-hydroxybenzophenones and the 2-(2'-hydroxyphenyl) benzotriazoles. Excited-state quenchers are exemplified by hindered-amine light stabilizers and certain nickel chelates. While the use of UV absorbers and excited-state quenchers are efficacious, the high cost of these materials argues for a reduction in the amount of such materials needed to achieve the effect sought.
An economical UV stabilizer system has been found which features reduced amounts of UV absorber, etc. in the system The system contains 0.1 weight percent to 1.0 weight percent hindered-amine light stabilizer (HALS), 0.25 weight percent to 1.5 weight percent of a UV absorber, i.e., a 2-(2'-hydroxy-phenyl) benzotriazole (HPBT), and a synergistic amount of ZnO and/or Mg(OH).sub.2. While such a system gives good UV stability, there is, as always, room for improvement.